Electronic circuits, such as integrated microcircuits, are used in a variety of products, from automobiles to microwaves to personal computers. Designing and fabricating microcircuit devices typically involves many steps, known as a “design flow.” The particular steps of a design flow often are dependent upon the type of microcircuit, its complexity, the design team, and the microcircuit fabricator or foundry that will manufacture the microcircuit. Typically, software and hardware “tools” verify the design at various stages of the design flow by running software simulators and/or hardware emulators, and errors in the design are corrected or the design is otherwise improved.
Several steps are common to most design flows. Initially, the specification for a new circuit is transformed into a logical design, sometimes referred to as a register transfer level (RTL) description of the circuit. With this logical design, the circuit is described in terms of both the exchange of signals between hardware registers and the logical operations that are performed on those signals. The logical design typically employs a Hardware Design Language (HDL), such as the Very high speed integrated circuit Hardware Design Language (VHDL). The logic of the circuit is then analyzed, to confirm that it will accurately perform the functions desired for the circuit. This analysis is sometimes referred to as “functional verification.”
After the accuracy of the logical design is confirmed, it is converted into a device design by synthesis software. The device design, which is typically in the form of a schematic or netlist, describes the specific electronic devices (such as transistors, resistors, and capacitors) that will be used in the circuit, along with their interconnections. This device design generally corresponds to the level of representation displayed in conventional circuit diagrams. Preliminary timing estimates for portions of the circuit may be made at this stage, using an assumed characteristic speed for each device. In addition, the relationships between the electronic devices are analyzed, to confirm that the circuit described by the device design will correctly perform the desired functions. This analysis is sometimes referred to as “formal verification.”
Once the relationships between circuit devices have been established, the design is again transformed, this time into a physical design that describes specific geometric elements. This type of design often is referred to as a “layout” design. The geometric elements, which typically are polygons, define the shapes that will be created in various materials to manufacture the circuit. Typically, a designer will select groups of geometric elements representing circuit device components (e.g., contacts, gates, etc.) and place them in a design area. These groups of geometric elements may be custom designed, selected from a library of previously-created designs, or some combination of both. Lines are then routed between the geometric elements, which will form the wiring used to interconnect the electronic devices. Layout tools (often referred to as “place and route” tools), such as Mentor Graphics' IC Station or Cadence's Virtuoso, are commonly used for both of these tasks.
With a layout design, each physical layer of the circuit will have a corresponding layer representation in the design, and the geometric elements described in a layer representation will define the relative locations of the circuit device components that will make up a circuit device. Thus, the geometric elements in the representation of an implant layer will define the regions where doping will occur, while the geometric elements in the representation of a metal layer will define the locations in a metal layer where conductive wires will be formed to connect the circuit devices. Typically, a designer will perform a number of analyses on the layout design. For example, the layout design may be analyzed to confirm that it accurately represents the circuit devices and their relationships as described in the device design. The layout design also may be analyzed to confirm that it complies with various design requirements, such as minimum spacings between geometric elements. Still further, the layout design may be modified to include the use of redundant geometric elements or the addition of corrective features to various geometric elements, to counteract limitations in the manufacturing process, etc.
In particular, the design flow process may include one or more resolution enhancement technique (RET) processes. These processes will modify the layout design data, to improve the usable resolution of the reticle or mask created from the design in a photolithographic manufacturing process. One such family of resolution enhancement technique (RET) processes, sometimes referred to as optical proximity correction (OPC) processes, may add features such as serifs or indentations to existing layout design data in order to compensate for diffractive effects during a lithographic manufacturing process. For example, an optical proximity correction process may modify a polygon in a layout design to include a “hammerhead” shape, in order to decrease rounding of the photolithographic image at the corners of the polygon.
After the layout design has been finalized, it is converted into a format that can be employed by a mask or reticle writing tool to create a mask or reticle for use in a photolithographic manufacturing process. Masks and reticles typically are made using tools that expose a blank reticle or mask substrate to an electron or laser beam (or to an array of electron beams or laser beams). Most mask writing tools are able to only “write” certain kinds of polygons, however, such as right triangles, rectangles or other trapezoids. Moreover, the sizes of the polygons are limited physically by the maximum beam (or beam array) size available to the tool. Accordingly, larger geometric elements in the layout design, or geometric elements that are not right triangles, rectangles or trapezoids (which typically are a majority of the geometric elements in a layout design) must be “fractured” into the smaller, more basic polygons that can be written by the mask or reticle writing tool. This process sometimes is referred to as “mask data preparation.”
Once a layout design has been fractured into shots, then the fractured layout design data can be converted to a format compatible with the mask or reticle writing tool. Examples of such formats are MEBES, for raster scanning machines manufactured by ETEC, an Applied Materials Company, and various vector scan formats for Nuflare, JEOL, and Hitachi machines, such as VSB11 or VSB12. The written masks or reticles then can be used in a photolithographic process to expose selected areas of a wafer to light or other radiation in order to produce the desired integrated circuit devices on the wafer.
To meet the demand for more powerful microcircuits, designers have regularly increased the average density of devices in a conventional microcircuit. For example, the area that might once have contained 100 transistors may now be required to contain 1,000 or even 10,000 transistors. Some current microcircuit designs call for microcircuit devices to be packed so closely that it may be difficult to properly manufacture adjacent device components in a single lithographic process. For example, a current microcircuit design may specify a series of parallel conductive lines positioned so closely that a conventional mask writer cannot resolve the pitch between the lines.
To address this issue, the structures in a layer of a microcircuit device are now sometimes formed using two or more separate lithographic processes. This technique, referred to as “double patterning,” partitions a layout design into two or more groups, each of which is then used to form a complementary lithographic mask pattern. Thus, if a layout design calls for a series of closely-spaced parallel connective lines, this target pattern may be partitioned so that alternating lines are actually formed by different masks in separate lithographic processes.
Because this “double patterning” technique typically is employed to ensure a minimum separation between adjacent structures in a microcircuit layer, the proximity relationships between the pieces of a target pattern may be used to define the partition. For example, a user may specify that pairs of edges in the target pattern must be imaged by different masks. This “separation directive” is then employed by a decomposition function to partition the target pattern so that new mask pattern conforms to the constraints given by the separation directive. Typically, this will require cutting the polygons making up a target pattern into segments. It is often difficult, however, to determine how the polygons in the target pattern should be segmented, i.e., where to cut the polygons. Ideally, the polygons should be segmented so that the resulting mask patterns will satisfy the separation directive while creating the smallest number of different polygon segments.